Re: want to clarify ways to "export" a make variable

[Paul Smith]

On Mon, 2015-04-20 at 07:08 -0400, Robert P. J. Day wrote:
>   just want to make sure i understand the details of exporting
> variables in gnu make.

It's trivial to test; write a makefile:

  all: ; @echo $$VAR

then set VAR in different ways and see which ones cause this to echo the
value.

>   first, if i set an environment variable either in the shell with:
> 
>   $ VAR=value
>   $ export VAR
>   $ make ...
> 
> or as a one-time thing on the make command line with:
> 
>   $ VAR=value make ...
> 
> is it true that that variable VAR is now part of the make invocation
> environment and through all sub-make invocations?

Yes.

>   also, if i do the above, in any part of that invocation, i can
> reassign the value of VAR, that new value would now be part of the
> environment at that level and all further sub-makes, and there is of
> course no need to "export" that variable again -- once it's part of
> the environment, it stays in the environment (short of explicitly
> "unexport"ing it). still good so far?

Yes.

>   next, i can always explicitly add a variable to the environment
> within a Makefile with:
> 
>   export VAR
> 
> but if that variable is *already* in my environment from the command
> line, the make "export" directive is superfluous -- again, doesn't
> hurt, just unnecessary.

Correct.

>   finally, if i change the origin of the variable to:
> 
>   $ make VAR=value ...
> 
> is that variable still part of the environment?

Yes.  See the GNU make manual "Communicating variables to sub-makes":

        Except by explicit request, `make' exports a variable only if it
        is either defined in the environment initially or set on the
        command line, and if its name consists only of letters, numbers,
        and underscores.